Pretty much the causes are fried or misprogrammed USB interface chip.
And if I upload a sketch to do a Serial.println("Hello!");, that works just fine, and I see the test come up in the Serial Monitor.
The driver was downloaded directly from the Prolific website.
In PuTTY, I get the same thing where I see nothing when I type, although I see the TX and RX lights on the board blink when I type.